1. Field of the Invention
The present invention relates to a method for improving productivity of aerobic culture by supplying oxygen using a sintered metal membrane in aerobic fermentation of useful substances.
2. Background Art Relating to the Invention
In fermentation industry, aerobic culture has been frequently conducted, wherein oxygen is supplied by aeration with stirring. In a conventional culture tank, oxygen is supplied to fungus bodies by aeration with stirring wherein air supplied from an aeration line equipped in the culture tank is finely dispersed by stirring blades. In this process, however, a problem of insufficient oxygen supply arises when oxygen consumption by fungus bodies is large due to a high fungus body concentration or a high production rate of a useful substance by the fungus bodies.
Thus, in order to improve oxygen supply to a culture liquid, use of oxygen instead of air as the supplying gas has been examined. However, there are problems that facilities for generating oxygen and maintenance thereof are required and a gas having a high oxygen concentration relative to air is discharged because sufficient oxygen supply is impossible by the conventional aeration with stirring even when oxygen is supplied.
Moreover, for the purpose of finely dispersing supplied air in an aerating stirrier, oxygen-supplying ability can be improved by increasing power for stirring. In this case, however, cavitation occurs when power for stirring is increased beyond a limit, so that the applied power has not been utilized efficiently for oxygen supplying.
In addition, a method of increasing aeration or inner pressure is also known for improving oxygen supplying, but the method is accompanied by problems that a high pressure affects the metabolism of the fungus bodies and also a large investment is necessary for increasing the capacity of a compressor.
Moreover, equipments for improving oxygen supply by circulating a liquid have come into practical use, such as a fine bubble pump (e.g., JP-A-6-193600 and JP-A-6-330888), a static mixer (e.g., JP-A-5-15753), a fine bubble nozzle (e.g., JP-A-9-201520). However, there are problems that the circulation of a liquid in a culture tank results in insufficient washing in a circulating line which causes bacterial contamination, and a large apparatus is required as the pump for circulation due to a large quantity of circulation in culture of the industrial level. Also, even if the static mixer or nozzle which is placed inside is used, a problem of bacterial contamination due to insufficient washing arises because the mixing part is closed in the line.
Furthermore, in a culture process using a sintered metal element (JP-A-61-56070), when air, oxygen-rich air, or oxygen gas is dispersed and supplied to a culture tank as fine bubbles using a sintered metal element, a large improvement of oxygen supplying ability is observed but there is a problem that solid matter adheres to the membrane during the culture and thus aeration cannot be continued because of increase of pressure loss, even when the aeration is conducted using the sintered metal element instead of the aeration line conventionally employed.
In general, ammonia gas for supplying nitrogen source and for controlling pH is supplied to a culture tank through a diffusing pipe for aeration in a usual aerobic culture for the purpose of efficient dispersion using stirring blades (e.g., Malcolm V. Bartow, Chemical Engineering, July, 70 (1999), Daniel I. C. Wang et al., Fermentation and Enzyme Technology, 230 (1979), and Apparatus and Instrument in Bioindustry, 10, ed. by Joji Takahashi, Japan Bioindustry Association (1987)). From the reason of avoiding bacterial contamination and inhomogeneity of pH and existing a possibility of back flow of a culture liquid at non-working time due to the high solubility of ammonia gas, it is rare to supply ammonia gas alone through a pipe other than a diffusing pipe for aeration. However, when aerobic culture is conducted with the change of the diffusing pipe employed in a conventional process for aerobic culture into a sintered metal element and ammonia is supplied, the present inventors have found for the first time that there arises a problem that solid matter adheres due to the roughness of the membrane surface and pressure loss at aeration increases, whereby it becomes impossible to continue the aeration under the discharge pressure of a current compressor and the use of the membrane should be stopped in the middle of the culture.
Accordingly, an object of the present invention is to develop a culture apparatus and culture process capable of improving an oxygen supplying rate by 60% or more and supplying ammonia simultaneously and also capable of conducting culture without problems of the membrane occlusion and bacterial contamination, by equipping a current aerating stirrier with a sintered metal membrane as a diffusing pipe.
This and other objects of the present invention have been accomplished by a process for aerobic culture wherein ammonia and oxygen are simultaneously supplied, which comprises: dispersing and supplying air, oxygen, or a mixed gas thereof into a culture tank through a sintered metal membrane attached to the end of a diffusing pipe, and simultaneously, dispersing and supplying ammonia or a mixed gas of ammonia and air into the culture tank through a diffusing pipe other than the above diffusing pipe.